Broadly, the invention relates to density measurements of liquids. More specifically, the invention is directed to measuring the density of a flowing liquid.
There are many industrial operations and processes in which the success of the operation depends on reliable measurement of the density of a liquid. An example of such an operation is the cementing of a borehole for an oil well. In this operation the cement, as a slurry, is injected into an annulus formed between the pipe casing and the wall of the borehole. When the slurry reaches the desired location in the borehole it is allowed to harden.
To achieve a satisfactory job in oil well cementing, it is extremely important to closely regulate the density of the cement slurry during the injection operation. The density of a cement slurry is a measure of the amount of water used with a sack of dry cement material. The amount of water in the slurry controls many physical properties of the cement. For example, too much water can increase the thickening and setting time of the cement beyond a desirable period. An excess of water in the slurry can also cause shrinkage which will weaken the set cement. In addition, too much water can cause pockets of water to form behind the pipe casing. By contrast, if the slurry contains too little water it will increase initial viscosity and thixotropy of the cement and cause plug flow or a preliminary flash setting.
A conventional instrument widely used in oil field work to measure cement slurry density is called a cement densitometer. The instrument is made up essentially of three elements, namely, a radioactive source, a radiation detector, and an automatic recorder. Briefly, the operation of the instrument involves passing gamma rays from the radioactive source through the cement slurry stream. Part of the gamma radiation is absorbed by the slurry and the unabsorbed portion is measured by a gamma ray detection cell. The radioactive energy absorbed by the slurry is proportional to the slurry density. The energy readings are continuously recorded on a strip chart, with the chart being calibrated to read directly as slurry weight, i.e. in pounds per gallon v. time.
The cement densitometer described above has certain disadvantages. One drawback of this instrument is that it can give different readings for cement slurries which have essentially the same density. The reason for the variance is that the additive content of each slurry composition may vary. For example, some slurry compositions may contain cellulose materials, which function as retarder compositions. Other slurry compositions may contain chloride compounds, which act as accelerator compositions. When the gamma radiation passes through each of the slurries mentioned above the actual absorption will be different for the cellulose material than it is for the chloride compounds. The result is that the density reading on the strip chart may be quite different for the two slurry compositions, but the true density may be very nearly the same. To obtain an accurate density reading, therefore, the instrument must be calibrated for each slurry composition which is measured.
The densitometer described above has another disadvantage in that the operation of the instrument is sufficiently complex to require a highly skilled operator. Another disadvantage of this instrument is the possibility of exposure of the operator to harmful radiation.